n. The act of wearing keys, colored handkerchiefs or other objects on the body to indicate sexual interests. Worn on left to indicate you're a bottom, on the right to indicate you're a top. There are several contradictory lists saying what each color code indicates. Mostly used in the gay male culture, and has branched out to lesbian and BDSM cultures, but may be dying out.
verb. to flag

He was flagging green, but I couldn't tell if it was hunter green or forrest green without my color chips.